1. Field of the Invention
This invention relates to a process for producing a pitch for the manufacture of high-performance carbon fibers, especially suitable for the manufacture of ultra high-performance carbon fibers, together with a pitch for the manufacture of general-purpose carbon fibers from a single heavy oil raw material of coal or petroleum origin.
2. Description of the Prior Art
Carbon fibers are conventionally classified into high-performance carbon fibers and general-purpose carbon fibers based on its mechanical strength. That is, carbon fibers having the strength of approximately 200-350 Kg/mm.sup.2 and modulus of elasticity of approximately 10-40 ton/mm.sup.2 are classified into the high-performance carbon fiber. These are directed to such applications as special parts material for rockets or aircraft, golf clubs, tennis rackets, fishing rods, and the like. On the other hand, those having the strength of approximately 70-140 Kg/mm.sup.2 and modulus of elasticity of approximately 3-10 ton/mm.sup.2 are classified into the general-purpose carbon fibers. They are used, for example, as thermal insulators, antistatic materials, sliding materials, filters, packings, and the like.
The recent expansion of the field to which carbon fibers are applied and the technological advance in the applications, however, require a further improvement in the mechanical strength of these materials. For example, ultra high-performance carbon fibers having the strength of the order of 300-600 Kg/mm.sup.2 are demanded. There also exist a wide variety of applications for general-purpose carbon fibers depending on its quality and performance. A more economical way of manufacturing this grade of carbon fibers is also desired.
Development of a simple process for preparing high-performance carbon fibers, especially ultra high-performance carbon fibers, together with general-purpose carbon fibers, using a single cheap raw material, for instance, a heavy oil of coal or petroleum origin is desired. Especially, it is meritorious that if general-purpose carbon fibers can be prepared from the spent fractions of the heavy oil which could not be utilized for the manufacture of high-performance carbon fibers in the past and was hitherto considered as valueless materials. If a process for co-production of high-performance carbon fibers and general-purpose carbon fibers is developed, it would have merits not only of production of high-performance carbon fibers, but also of lower production cost of general-purpose carbon fibers. Such a process would also contribute the reduction of production cost of high-performance carbon fibers. Thus, the process would be of tremendous industrial significance. No processes for sufficiently achieving these objectives, however, have ever been proposed.
In the followings, the terms "high-performance" and "general-purpose" are occasionally abbreviated as "HP" and "GP", respectively.
The reason why a process for co-production of HP carbon fibers and GP carbon fibers has not yet been proposed is believed as follows: That is, the reason is greatly attributable to the great difference in requisite for the pitch to be used in the production of HP carbon fibers and requisite for the pitch to be used in the production of GP carbon fibers. In preparing the HP carbon fibers from a pitch, the spinning pitch must be a so-called mesophase pitch which contains, as a major component, the substance exhibiting an optically anisotropic phase when examined on a polarizing microscope near the ambient temperature. On the contrary, the pitch for the production of GP carbon fibers is an entirely optically isotropic pitch which does completely not contain the optically anisotropic portion.
This mesophase is a kind of liquid crystals which is formed when a heavy oil or a pitch is heat-treated, and its optically anisotropic character is due to an agglomerated layered structure of thermally polymerized planar aromatic molecules. Further, the agglomerated layered structure of planar aromatic molecules have a property easily to form orientation and the property just mentioned above has an important role when carbon fibers are prepared from the pitch. When such mesophase pitch is subjected to melt spinning, the planar aromatic molecules are aligned to the direction of the fiber axis due to the stress exerted to the melt as it passes through a nozzle hole, and this oriented structure can be kept without being disrupted throughout subsequent steps to render it infusible and carbonization steps, and therefore, high-performance carbon fibers with high tensile strength and high modulus of elasticity and having good orientation can be obtained. Therefore, when the production of high-performance carbon fibers with high tensile strength and high modulus of elasticity is desired, it is necessary to use an optically anisotropic pitch as the raw material. Accordingly, it is an important task in the art that how the optically anisotropic pitches with good spinnability are prepared.
It is to be understood that the term "mesophase" used in the art is a synonym of "optically anisotropic phase" or "optically anisotropic portion" and the term "mesophase pitch" is a synonym of "optically anisotropic pitch".
In most of the cases, however, this optically anisotropic portion is different from the non-oriented, optically isotropic portion in its viscosity, specific gravity, etc. A pitch containing, for example, a small amount of optically anisotropic portion mixed with optically isotropic portion, even if heated to melt at a temperature at which the optically isotropic portion becomes a viscosity to be easily spun, cannot be spun in a stable manner because of the existence of the small amount of optically anisotropic portion having a considerably high viscosity at this temperature. For the manufacturing of general-purpose carbon fibers from an optically isotropic pitch, therefore, the absence of optically anisotropic portion in the optically isotropic pitch is imperative. For the preparation of GP carbon fibers, it is therefore very important to suppress the formation of optically anisotropic portion in the optically isotropic pitch.
Although pitches for the manufacture of HP and GP carbon fibers are common in that both are spinning pitches, they are completely different from each other in that the one allows the existence of optically anisotropic portion while the other does not. This would be the reason that no attempts have ever been undertaken to develop a process which can produce both of these two pitches at the same time.